In the related art, a typical objective lens for an endoscope has a depth of view in a wide range of about 5 mm to 100 mm at an object side, instead of having a focusing function. The depth of view is a distance range between an object and an end surface of an objective optical system in which a clear image of the object can be acquired. An endoscope equipped with such an objective lens provides an image by mainly using a solid-state image acquisition element, such as a CCD. In recent years, in order to meet the demands for high-quality endoscopic images for improving diagnostic accuracy, high-resolution image acquisition elements, such as CCDs, have been developed.
If a high-resolution image acquisition element is used, the F number (Fno.) of the objective lens needs to be reduced so as to avoid image deterioration caused by diffraction, and the focal length of the objective lens needs to be increased in accordance with the increased size of the image acquisition element, resulting in a reduced depth of view of the objective lens. In order to ensure a depth of view as high as that in the related art, a known observation lens for an endoscope achieves a wide angle by reducing the focal length by using, for example, an aspherical lens.